Unsaturated carboxylic acids such as acrylic acid and methacrylic acid are industrially important as starting materials for various synthetic resins, coating materials and plasticizers. Nitriles, such as acrylonitrile and methacrylonitrile, are industrially important intermediates for the preparation of fibers, synthetic resins, synthetic rubbers, and the like. Such unsaturated carboxylic acids and nitriles can be produced by catalytic (amm)oxidation of lower (i.e., C2-C8) alkanes and alkenes, such as ethane, ethane, propane, propene, butane (including n- and iso-butane), butane (including n- and iso-butene) and pentane (including n- and iso-pentane) and pentane (including n- and iso-pentene).
For example, the currently practiced commercial process for the production of acrylic acid involves a two-step catalytic vapor phase oxidation reaction using an alkene, propene, as the hydrocarbon starting material. In the two-step oxidation reaction, propene is converted to acrolein over a suitable mixed metal oxide catalyst in the first step. In the second step, acrolein product from the first step is converted to acrylic acid using a second suitable mixed metal oxide catalyst. In most cases, the catalyst formulations are proprietary to the catalyst supplier, but the technology is well established. Furthermore, it is known to include additional starting materials, including additional reactants, such as molecular oxygen and/or steam, and inert materials, such as nitrogen and carbon dioxide, along with the hydrocarbon starting material that is fed to such two-step oxidation processes. See, for example, U.S. Pat. No. 5,218,146, which discloses a two-step catalytic vapor phase oxidation process for conversion of propene to acrylic acid. In the disclosure of U.S. Pat. No. 5,218,146, carbon dioxide is fed to the two-step oxidation process in an amount of from 3% to 50% by volume, based upon the total volume of the starting materials, which also include propene and molecular oxygen. There is, however, no correlation provided, expressly or implicitly, in U.S. Pat. No. 5,218,146 between the amount of carbon dioxide which is fed to the process and the yield of acrylic acid product.
The most popular method for producing nitrites is to subject an alkene (olefin), such as propene or isobutene, to a catalytic reaction with ammonia and oxygen in the presence of a suitable catalyst in a gaseous phase at a high temperature. There are various known catalysts suitable for conducting this reaction and, while many of the catalyst formulations are proprietary to the catalyst supplier, this technology is also well established. Furthermore, it is known to include additional starting materials, including additional reactants, such as molecular oxygen and/or steam, and inert materials, such as nitrogen and carbon dioxide, along with the hydrocarbon and ammonia starting materials that are fed to such two-step ammoxidation processes.
In view of the lower price of alkanes (for example, propane and isobutene) in comparison to alkenes (for example, propene and isobutene), attention has been drawn to the development of catalysts and processes for the production of unsaturated carboxylic acids and unsaturated nitrites in a single-step vapor phase (amm)oxidation process using the cheaper alkane as the hydrocarbon starting material. For example, catalysts capable of catalyzing the single-step oxidation of propane to acrylic acid in yields up to 52% have been developed and continue to be improved.
In addition, some refinements to the single-step oxidation process itself have been developed and further improvements to the single-step oxidation process continue to be sought and welcomed by industry. For example, it is known to include additional starting materials, including additional reactants, such as molecular oxygen and/or steam, as well as inert materials, such as nitrogen and carbon dioxide to act as diluents or heat moderators, along with the hydrocarbon starting material that is fed to the one-step oxidation process.
For example, U.S. Pat. No. 6,646,158 which states that carbon dioxide may be fed to the oxidation process in amounts greater than 5% by volume, based on the total volume of the feed gases, but no examples are provided that include feeding carbon dioxide to the disclosed process. Thus, carbon dioxide is not required for this process and no conclusions may be drawn from U.S. Pat. No. 6,646,158 regarding the efficacy of carbon dioxide as a diluent or heat moderator. In addition, U.S. Pat. No. 6,693,059 discloses the possibility of feeding a diluting gas, such as nitrogen, argon, helium or carbon dioxide, in an amount of from 0% to 20%, by volume, to a single-step oxidation process which converts propane to acrylic acid. This patent, however, is focused on the catalyst composition and activity and no examples are provided that include feeding carbon dioxide to the single-step oxidation process. O. V. Krylov et al., in “The regularities in the interaction of alkanes with CO2 on oxide catalysts,” Catalysis Today 24 (1995) 371-375, disclose the use of carbon dioxide as a non-traditional oxidant in the oxidation of methane, ethane and propane, but the products include only synthesis gases (hydrogen and carbon monoxide) and simple oxydehydrogenation products such as alkenes, without production of unsaturated carboxylic acids or nitrites. Thus, none of these prior disclosures explore or discuss the use of carbon dioxide as a feed component to single-step (amm)oxidation processes for increasing the production of (amm)oxidation products, including unsaturated carboxylic acids and nitrites.
Thus, the chemical industry would welcome further improvements to increase the yields of single-step (amm)oxidation processes for the conversion of one or more C2 to C8 alkanes to valuable (amm)oxidation products, including unsaturated carboxylic acids and nitrites.